Messi, 26, comes into the tournament after a frustrating season at Barcelona, where he was troubled by issues including niggling injury, underwhelming form, a tax fraud enquiry and long-running speculation over his future at the Catalan club.

Speaking ahead of Group F's opening game against Bosnia-Herzegovina on Sunday, Messi's Barca clubmate told Argentine paper Ole that the four-time Ballon d'Or winner was in good spirits, which suggested he was ready to play at his best in the tournament.

"Leo is happy and that is a great sign," Mascherano said. "When he is content is when he performs best."

Mascherano said having such a star as captain did not mean Argentina's other players did not also need to be at their maximum too.

"We cannot excuse ourselves of responsibility," he said. "We have to make him perform at his best. It will depend on what we give him as a team."

The former Liverpool player said his take on Brazil's shaky 3-1 win over Croatia in the World Cup's opening game -- which featured a controversial penalty awarded to the tournament hosts -- was that less-fancied teams could cause surprises as the competition progresses.

"If [Brazil], as the big favourites and being hosts had it tough...," Mascherano said. "With the excitement [over the penalty] we missed the fact that there are countries not so talked about which have good players. Bosnia have three or four world class players and they play good football. This game showed us how challenging everything will be."